Why does "I'm sure" used in a sentence sometimes reduce apparent certainty?

Solution 1:

The reason that the second statement is perceived as less certain than the first has to do with the fact that we are used to hearing statements like "I'm sure" added in cases where the speaker only believes it to be true without having witnessed proof. Consider this third statement:

I know for a fact that this road is closed during football games.

This is another sentence where the perspective of the speaker is introduced, but it carries as much or more perceived certainty as the first statement. This is because what we assume is true for the first statement and is false for the second statement—the presence of evidence of the fact—is stated explicitly in the third statement. If we all only added "I'm sure" to statements when we indeed were sure based on proof, we would not experientially attribute less certainty to statements constructed this way.